1. Field of the Invention
The present invention relates to a lifting apparatus for use in assembling a building at an elevated spot, painting and the like at the elevated spot, lifting operators or materials upward for operation at the elevated spot or loading and unloading disused building materials at the building work, particularly to the lifting apparatus capable of lifting a platform to the elevated spot irrespective of the short length of a lifting mechanism at folding state and of preventing wires, chain for connecting each boom from being broken.
2. Prior Art
There has been employed a lifting apparatus for assembling, painting, repairing a building, and the like at an elevated spot, which apparatus is capable of lifting or lowering for loading operators or building materials and the like thereon or unloading the disused materials therefrom.
There has been employed a pantograph type telescopic mechanism, i.e. scissors type comprising a first pair of arms pivotally connected with each other at a central portion thereof and plural pairs of arms connected with the first pair of arms. In this apparatus, it was necessary to lengthen the length of the pairs for increasing the maximum height of the apparatus. Hence, if an apparatus capable of lifting upward as high as possible is-designed, it was necessary to employ a plurality of paired pantographs, which entails increasing the height of the apparatus when folded whereby it is more troublesome for an operator to get thereon or thereoff or to load materials thereon or unload materials therefrom.
There have been various proposed arrangements to solve the problems set forth above, for example the one as disclosed in U.S. Pat. No. 3 820 631. In a mechanism as proposed by this patent, a lower boom and an upper boom are respectively capable of moving linearly into a middle boom, the lower boom is pivotally mounted on a chassis at the end thereof, the upper boom is pivotally mounted on a platform at the end thereof, and these booms are assembled to form an X-shape. In this mechanism, inasmuch as the length of the boom per se becomes long, the height of the platform when folded can be decreased and the platform can be raised to the elevated spot.
However, in this known mechanism, inasmuch as the mechanism for extending the lower boom and upper boom from the middle boom comprises a screw and a thread for engaging with this screw, the telescopic moving speed of the lower and upper booms relative to the middle boom is slow, and hence the platform cannot be moved quickly. Furthermore, since the sliding motion of the lower boom and the upper boom is made by a bevel gear provided at the central portion of the middle boom, the entire length of the combination of the lower boom and the upper boom extending from the middle boom reaches a length only half as long as the middle boom, and hence the mechanism has such a structure that the platform cannot be raised as high as possible.
There has also been proposed a mechanism wherein another boom is inserted into a boom to extend the length thereof so that the entire length thereof is lengthened. For example, in FIG. 4 of Japanese Patent Laid-Open Publication No. 53-19556, lower and upper booms respectively having small diameters are inserted into a middle boom having a large diameter so that the lower and upper booms inserted into the middle boom are pulled out to lengthen the entire length of the booms, whereby the platform is raised high.
However, in this latter mechanism, there is no mechanism for synchronizing the amount of extension and contraction of the lower boom pulled out from the middle boom with that of the upper boom as also pulled out from the middle boom. The lower and the upper booms move individually relative to the middle boom. The amount of extension and contraction is restricted by a link mechanism comprising bars, and hence the complete synchronization of the lower and upper booms relative to the middle boom cannot be achieved. Accordingly, the lower and upper booms cannot be connected to the platform by a pin and the like and a non-synchronized error of the amount of the extension and contraction between the lower and upper booms relative to the middle boom can be absorbed by rollers contacting the chassis and the platform. Hence, the platform is liable to swing because of accumulation of jolt caused by many supporting fulcrums and reception of the rolling motion by the roller. As a result, the mechanism is liable to swing due to wind and the like and is unstable, thereby causing the operator to feel anxious.
To solve the drawbacks set forth above, there has been proposed a mechanism as disclosed in Japanese Patent Application No. 56-41289. In this application, lower and upper booms are inserted into a middle boom while both the lower and upper booms are connected by coupling means at one end thereof and the movable direction of the coupling means can be turned by a turning means pivotally mounted on the middle boom.
In this latter mechanism, inasmuch as the upper boom is pulled out from the middle boom at the same time when the lower boom is extracted from the middle boom and the movable amount of the lower and upper booms are restricted by the coupling means, the movable amount of the lower boom equals that of the upper boom, and hence a pair of middle booms supported by the lower and upper booms at the center thereof turns in an X-shape to thereby raise the platform vertically upward. In this mechanism, since the lower and upper booms are accommodated in the middle boom, it is possible to stretch the entire length of the booms about three times as long as the length of the middle boom when the lower and upper booms are respectively pulled out, hence the platform can be raised high.
The above lifting apparatus is characterized in comprising a pair of X-shaped middle booms having upper and lower openings, upper and lower booms being pulled out from the middle boom through the upper and lower openings wherein the lower boom is connected to the chassis and the upper boom is connected with the platform. The mechanism has an X-shape if viewed from the side thereof. In this mechanism, it is possible to decrease the height of the mechanism when folded such as a scissors-type mechanism and secure the platform against swinging since the respective distal ends of the lower and upper booms are connected by the pins with the chassis and the platform, which enhances the safety. Furthermore, inasmuch as the lengths of the lower and upper booms can be substantially the same as the length of the middle boom, there are many advantages such as the platform can be raised high and the height for raising the platform can be increased compared with the entire lengths of the booms when folded.
However, there occurred the following first problem. That is, the conventional X-type lifting apparatus has a structure to extend and contract in three stages since the lower and upper booms are inserted into the middle boom. To increase the height of the platform, it is necessary to design the length of the middle boom to be set to be longer. Thus, the platform can be raised high by lengthening the middle boom. However, the entire length of the chassis accommodating the middle boom is lengthened, which entails drastic change in the design of the lifting apparatus. Hence, the height of the lifting apparatus to be raised is determined by the length of the middle boom and the entire length of the chassis which are great obstacles.
Accordingly, there is desired a development of the lifting apparatus capable of lifting the platform as high as possible while permitting the middle boom to have the same length as the conventional mechanism.
Next, in the aforesaid apparatus, there occurred the following second problem. That is, it was necessary to connect the middle boom to the upper and lower booms by wires or chains or the like for synchronizing the upper and lower booms relative to the middle booms. The length of the lower boom pulled out from the middle boom is synchronous with the movable length of the upper boom by connecting the upper end of the lower boom and the lower end of the lower boom with the wires, chains and the like, whereby the lifting mechanism is always maintained to form the X-shape. Although it is very simple in this arrangement to synchronize with use of wires, chains and the like, it was necessary to set the safety load toward the tensile stress in view of preventing an accident.
In setting the safety load, the safety load is insignificant when the ratio of height of the lifting mechanism when folded relative to that when raised at the maximum is small. However, if the same ratio is large, the design of the safety load becomes a very significant matter.
That is, when the platform is raised to an elevated spot, the angle of inclination of the booms relative to the horizontal is large and a component of the force of the load applied to the platform is not large. Hence, the tensile strength applied to the wires for connecting the lower boom to the upper boom is not excessive. However, when the platform is lowered, the angle of inclination of the booms relative to the horizontal becomes small and the component of the force of the load applied to the platform becomes large. This component of the force of the load is applied directly to the wires or chains serving for synchronization, hence the tensile strength becomes very strong. Accordingly, if the safety factor of the load applied to the wires, chains or the like is set to be small, there is a likelihood of generating such an accident load that the wires, chains or the like are broken by the component of the force. When the wires, chains or the like for connecting the lower boom with the upper boom are broken, the platform lowers suddenly which can cause injury or damage.
Accordingly, wires, chains or the like having low safety factor do not generate any problem when they are used for synchronization at the state where they are raised high but they become one of the reasons of generating accidents when the platform is lowered which increases the component of the force of the load, thereby possibly breaking the wires, chains or the like.
To prevent generation of such accidents, it is preferable to increase the safety factor and set the safety load of the wires, chains or the like to a large value. If the wires, chains or the like becomes thick to increase the safety factor, the wires becomes too thick, in the worst case, to function as the lifting apparatus due to deterioration in flexibility thereof.